1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method that droplets are ejected from nozzles to form images on a recording medium.
2. Description of the Related Art
Image forming apparatuses with inkjet systems form images on a recording medium by ejecting ink from nozzles provided to a print head. In such image forming apparatuses, when second ink droplets are deposited so as to overlap first ink droplets that are deposited first on the recording medium, if the first ink droplets remain unsettled on the recording medium, the ink droplets mix together in the areas where the second ink droplets and the unsettled first ink droplets overlap, which blurs the original shape of the dots, creates mixed colors when inks of different colors are used, and causes degradation in the image. The mixing together of ink droplets deposited on the recording medium is referred to as a “deposition interference” or a “landing interference.”
In view of this, in order to prevent deposition interference or the smearing or mixing of ink on the recording medium, a technique has been proposed that uses an electro rheological fluid (see Japanese Patent Application Publication Nos. 2-212149, 5-4342, and 5-4343).
Japanese Patent Application Publication No. 2-212149 discloses a technique for applying an electric field to the recording medium on which the recording liquid is deposited after the recording liquid that has electro rheological effects is deposited on the recording medium, so as to suppress permeation of the recorded dots and to prevent smearing or decreases in concentration.
Japanese Patent Application Publication No. 5-4342 discloses a technique that droplets of a recording liquid having electro rheological effects, which are formed by a recording head (print head), are deposited onto an intermediate transfer medium with an electric field formed on the surface, so as to increase the viscosity of the droplets on an intermediate transfer medium. Since the droplets in a thickened state are transferred onto a transfer medium (recording medium), it is possible to prevent the recorded dots from expanding excessively or from mixing. Also, in Japanese Patent Application Publication No. 5-4342, it is described that the strength of the electric field applied to the recording liquid in the recording head should be adjusted to zero or the viscosity of the recording liquid when the electric field is applied should be adjusted to no more than a specific value, in order to perform droplet ejection with the recording head in a stable condition.
Japanese Patent Application Publication No. 5-4343 discloses a technique that droplets of a recording liquid having electro rheological effects, which are formed by a recording head, are deposited onto a transfer medium with an electric field formed on the surface, so as to increase instantaneously the viscosity or yield value of the recording liquid. Therefore, it is possible to prevent smearing, deterioration, and color mixing in the recording dots. Also, Japanese Patent Application Publication No. 5-4343 describes that the strength of the electric field applied to the recording liquid in the recording head should be adjusted in the same manner as in Japanese Patent Application Publication No. 5-4342.
Furthermore, in those references, a method is described in which a corotron device and other electric charging devices are used as an electric field formation device to provide an electric charge to the surface of transfer medium or intermediate transfer medium depositing the recording liquid so that an electric field is formed on the surface thereof. In addition, another method is also described in which a pair of electrodes is provided on both sides of the transfer medium so that a direct current voltage is applied to the pair of electrodes to form an electric field.
However, it has been made clear though the experiments of the inventors that the viscosity of the liquid deposited on the recording medium does not increase sufficiently even if an electric field is formed by the electric field formation devices disclosed in Japanese Patent Application Publication Nos. 2-212149, 5-4342, and 5-4343, which results in deposition interference between droplets on the recording medium, and in permeation smearing or color smearing in which the deposited droplets permeate and smear on the recording medium.
Additionally, the viscosity of the recording liquid with which unsatisfactory ejection occurs in the recording head is far less than the viscosity at which deposition interference and the like between droplets on the recording medium can be prevented, and is about 1/100th to 1/1000th of the viscosity needed for the droplets on the recording medium to avoid deposition interference. Therefore, when applying an electric field to the droplets deposited on the recording medium, the image forming apparatus might be designed with regard to the thickness of the recording liquid in the recording head. However, although the necessity of adjusting the strength of the electric field applied to the recording liquid in the recording head is described in Japanese Patent Application Publication Nos. 5-4342 and 5-4343, the specific device for this purpose is not indicated specifically.